The dual in-line package (DIP) integrated circuit (IC) is widely used in electrical and electronic applications, including applications where reliability is crucial. In space communications and instrumentation networks or systems, as well as medical applications, the requirement of continuing reliability can be particularly strict. For that reason, it is desirable and generally necessary to test the circuit boards containing DIP IC's before the boards are incorporated into such a network or system.
Such testing often includes the connecting of the pins, or terminals, of the DIP to test equipment so as to monitor appropriate outputs. Because of the small, compact nature of the DIP IC, it is possible for two terminals to be inadvertently short-circuited during the testing procedure and, possibly, during actual operation. Such short-circuiting can result in serious damage to the circuitry, damage which might not be detected until after the circuit board is connected into the network or system, and the entire network or system is put into operation. The cost of remedying the damage at that point can often be tremendous, if such damage can be repaired at all.
In order to test DIP IC's with greater facility, various devices have been suggested. U.S. Pat. No. 3,506,949 discloses a connector which engages a DIP IC so that each pin of the DIP component can be extended by a conductor which can be more easily connected to a probe or other piece of test equipment. U.S. Pat. No. 3,506,949 discloses a connector having a plurality of parallel conductors, each such conductor passing through a separate channel in the connector, thereby isolating the conductors from each other. While substantially eliminating the possibility of conductors touching along their lengths, the reference fails to address the problem of two conductors being short-circuited by a test probe or by an accidental bending of the conductor ends, which may also result in short-circuiting.
U.S. Pat. No. 3,899,239 similarly recognizes the serious damage that might be caused by the shorting of adjacent connector wires during testing. In response to this problem, the reference provides eyelets at the ends of connector elements onto which a test probe can be hooked. Further, adjacent connectors are disposed at 90.degree. angles and each connector is insulated along its length except at the eyelet portions. Like U.S. Pat. No. 3,506,949, this reference addresses the problem of preventing short-circuiting but does not consider the problem of protecting the DIP circuitry when two connector wires are inadvertently short-circuited.
U.S. Pat. Nos. 3,914,007 and 3,551,878 also disclose clips, or connectors, which serve to extend the length of a DIP pin in order to facilitate testing. Neither of these references, however, confront the problem of preventing damage after there is a short across pin extenders. U.S. Pat. No. 3,551,878, in particular, does not suggest the inclusion of any elements which might protect the circuitry from the effects of short-circuiting and in fact suggests the opposite. According to this reference, it is recommended that the test connection be placed as close to the integrated circuit module as possible in order to "eliminate the electrical characteristics of the leads from the test system". The reference thus suggests that there be no elements along the connector leads which might affect the sensed output in any way.
The device disclosed in British Patent No. 1,521,614 relates to an integrated circuit chip test rack for use in automated testing where chip test points are automatically connected to a pair of conductive tracks at a terminal touch spot. Included is a diode which acts only as a switch, its state depending on the voltage applied thereto, which in turn determines if an associated input track or output track conducts. When the diode is forward biased, the input track conducts as a straight wire to provide an input testing signal on top of the biasing signal to the integrated circuit being tested. Conversely, a reverse biased diode appears as an open circuit which disconnects the input track and presents a high impedance to any output signal appearing on the associated output track. This diode does not operate to prevent a short circuit in the event adjacent integrated circuit (IC) terminals are simultaneously contacted by the automated test points. Specifically, with the diode forward biased, the corresponding signal of the input track is applied directly to all contacted IC terminals. Whereas, with the diode reverse biased, any contacted IC terminals would be connected directly to the associated output track. Thus, no short circuit protection is provided.
According to the British Patent, a substrate with its associated parts is provided at a testing station of an automatic testing apparatus; integrated circuit elements being fed in turn to the testing station at which each element is presented in a predetermined orientation. A test head carrying the substrate is advanced towards the element currently at the testing station. Thus, the only moving element is the integrated circuit having terminals which, if at all possible, may be electrically connected by misaligned terminal touch spots. Assuming arguendo that such a misalignment could occur, there is nothing in the British device that would prevent a short circuit between two chip terminals upon electrical connection with a terminal touch spot.
Fifty ohm resistances are included in the input/output tracks, providing matched termination to avoid the generation of unwanted reflections. In an automated testing mechanism such as the British device, a high frequency input signal (10 MHz) is used to test the IC terminals. Due to the high frequency of this signal, the input signal is electrically reflected from an associated terminal back through the input line, drastically distorting the desired test signal; i.e., the reflected signal sums with and subtracts from the original input signal. The fifty ohms provided in the input/output tracks is a value which only serves as a termination resistance to prevent reflections of the test signal and is insufficient to prevent a short circuit between any tracks which may, if at all possible, be electrically connected. In other words, to prevent damage to the IC chip, the current of the test signal must be kept below a value of approximately 10 milli-amps. Since the IC terminals are supplied with voltages of up to 10 v, a resistance of 100 ohms, (assuming two inputs are shorted together in the British device) will allow a current of 100 milli-amps to flow, thus destroying the IC.